This invention concerns anaerobic compositions, which have their primary utility in the area of polymerizable (curable) adhesives and sealants. Anaerobic compositions are commonly composed of curable unsaturated monomers, specifically acrylate ester monomers, in combination with peroxy polymerization initiators. While the peroxy initiator in the anaerobic composition is capable of polymerizing the monomer within a short time in the substantial absence of oxygen, such polymerization will not take place as long as the composition remains in adequate contact with oxygen.
The fact that the cure mechanism for anaerobic compositions is chemically blocked by atmospheric oxygen permits the composition to be stored in partially empty containers, preferably made of air permeable plastic such as low density polyethylene. Stored in such fashion, the anaerobic composition will remain in the liquid state. However, when placed between air impermeable surfaces, such as metal, glass, etc., the inhibiting influence of oxygen is lost and cure will commence in a relatively short time.
Two areas of particular interest in evaluating anaerobic compositions are the cure speed of the composition, and its ability to cure through gaps. For high speed production line applications where sealed or bonded articles cannot be stored for extended periods, and for equipment repair where prompt return to service is important, the highest possible rate of cure for the composition is desirable. (This assumes that the rapid cure speed does not produce overriding accompanying negative effects, such as undue embrittlement of the cured composition, or loss of other desirable properties.) Of additional and particular importance in anaerobic compositions is the cure through gap characteristic. As the gap (i.e., the space between the substrates to be sealed or bonded) becomes larger, the possibility of entrainment of atmospheric oxygen increases, and the area of the adhesive which is in contact with oxygen is also greater. Consequently, the inability to provide complete cure through large gaps has been a common problem with many anaerobic compositions.
Essentially all anaerobic compositions contain accelerators, compositions which do not destroy the shelf stability of the product but which accelerate the rate of cure once it has been initiated during use conditions. While these ingredients do increase the speed of cure of the products, they have provided little, if any, assistance in generating the ability to cure through gaps. Increased speed of cure also is produced in commercial practice by the use of surface primers or activators, chemical compounds which are applied to a surface prior to application of the anaerobic composition and which serve to dramatically increase the speed of cure. These compounds have not been added directly to the anaerobic composition due to their extreme activity and tendency to destroy the shelf stability of the product. Typical examples of such surface primers or activators are disclosed in, for example, U.S. Pat. Nos. 3,591,438 and 3,625,930. While the latter discloses thiourea as an effective accelerator, the disclosure is limited to cure through relatively small gaps. Various organometallic compounds are commonly used as such surface activators.
An anaerobic composition, or an anaerobic composition and a surface activator therefor, which provided improved speed of cure characteristics combined with the ability to cure through a gap, such as a gap of from about 5 to about 30 mils, would be a highly useful and desirable improvement in the area of anaerobic compositions, and a major advance in the art of sealing and bonding technology.